Semi-global matching (SGM) cost compression

ABSTRACT

This disclosure describes techniques for performing semi-global matching (SGM) path cost compression. In some examples, the techniques may perform disparity-dependent sub-sampling of a set of SGM path costs where the sub-sampling ratio is determined based on a candidate disparity level. The sub-sampled SGM path costs may be stored in a memory. When retrieved from memory, the sub-sampled SGM path costs may be interpolated to reconstruct the other path costs not stored in the memory. The reconstructed path costs may be used for further SGM processing. In further examples, the techniques may perform disparity-dependent quantization on the SGM path costs or the sub-sampled SGM path costs, and store the quantized SGM path costs in memory. The techniques of this disclosure may reduce bandwidth as well as reduce the memory footprint needed to implement an SGM algorithm.

This application claims benefit of U.S. provisional patent application Ser. No. 62/109,998, filed Jan. 30, 2015, which is herein incorporated by reference.

TECHNICAL FIELD

This disclosure is related to stereo matching techniques, and more particularly to techniques for implementing semi-global matching.

BACKGROUND

Semi-Global Matching (SGM) may achieve robust stereo correspondence results. However, the high on-chip memory usage of SGM to store aggregated SGM path costs in line buffer increases area. As a result, it is often impractical to implement in embedded platforms for high resolution images.

SUMMARY

A method includes, for each of a plurality of candidate disparity levels, calculating, with processing circuitry, a plurality of costs for a plurality of pixels in a scan region based on the respective candidate disparity level; calculating, with the processing circuitry, a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region based on the costs and the respective candidate disparity level; determining, with the processing circuitry, a sub-sampling ratio based on the respective candidate disparity level; sub-sampling, with the processing circuitry, the SGM path costs based on the determined sub-sampling ratio; and storing, with the processing circuitry, the sub-sampled SGM path costs for the respective candidate disparity level in a memory.

An apparatus includes processing circuitry configured to, for each of a plurality of candidate disparity levels calculate a plurality of costs for a plurality of pixels in a scan region based on the respective candidate disparity level; calculate a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region based on the costs and the respective candidate disparity level; determine a sub-sampling ratio based on the respective candidate disparity level; sub-sample the SGM path costs based on the determined sub-sampling ratio; and store the sub-sampled SGM path costs for the respective candidate disparity level in a memory.

A non-transitory computer-readable medium comprising instructions that, when executed by one or more processors, cause the processors to calculate a plurality of costs for a plurality of pixels in a scan region based on the respective candidate disparity level; calculate a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region based on the costs and the respective candidate disparity level; determine a sub-sampling ratio based on the respective candidate disparity level; sub-sample the SGM path costs based on the determined sub-sampling ratio; and store the sub-sampled SGM path costs for the respective candidate disparity level in a memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of a parallelogram block raster scan processing for forward SGM;

FIG. 2 is an embodiment of a disparity-dependent Semi-Global Matching (SGM) path cost sub-sampling for each pixel;

FIG. 3 is an embodiment of a bi-linear interpolation of unavailable SGM path costs;

FIG. 4 is an embodiment of a local cost guided bi-linear interpolation of unavailable SGM path costs;

FIG. 5 is an embodiment of a disparity-dependent SGM path cost quantization for each pixel; and

FIG. 6 is an embodiment of a system for performing SGM path cost compression according to this disclosure; and

FIG. 7 is an embodiment of a method 700 for SGM path cost compression.

FIG. 8 is a block diagram showing an example automobile system 800 that may implement the SGM cost compression techniques of this disclosure.

DETAILED DESCRIPTION

SGM achieves accurate distance measure for ADAS application. However, DDR memory bandwidth and high on-chip memory usages make it impractical to implement in embedded platform for high-resolution images. To tackle the DDR memory bandwidth issue in SGM, the parallelogram block raster scan processing and tile-based processing were proposed for forward pass and backward pass. However, these solutions cause memory issues. Accordingly, there is a need for a solution that would resolve the high on-chip memory issue in SGM.

FIG. 1 is an embodiment of a parallelogram block raster scan processing for forward SGM. In FIG. 1, k is the height of block. The forward SGM aggregation is done by aggregating SGM path costs, from neighboring left, top-left, top and top-right pixels. Therefore, the aggregated SGM path costs of every pixel in every k-th row are usually stored in a line buffer, for accessibility when processing the next row. The aggregated path costs for each pixel are stored, L_(r)(p, d), where d=d_(min)˜d_(max)−1, p=

, ↓, and

. For 2K width, d_(min)=0 and d_(max)=196. Thus, on-chip memory as large as 3 dir*2K*196*1.5 bytes>1720 Kbytes is needed. However, improved SGM path cost compression methods are proposed to reduce the on-chip memory size for SGM. For example, 1) disparity-dependent sub-sampling and interpolation may be used and/or 2) disparity dependent quantization and inverse quantization.

FIG. 2 is an embodiment of a disparity-dependent Semi-Global Matching (SGM) path cost sub-sampling for each pixel. As shown in FIG. 2, different sub-sampling ratios are applied depending on the disparity range to exploit unequal importance of disparity errors. In this example, SGM path costs between dmin and ds1 are not sub-sampled, those between ds1 and ds2 are sub-sampled by 2, and those between ds2 and dmax−1 are sub-sampled by 4. Note that it is also possible to divide disparity range more finely (e.g. >3 sub-sampling ratios) and coarsely (e.g. just 1 sub-sampling ratio, i.e. disparity independent).

FIG. 3 is an embodiment of a bi-linear interpolation of unavailable SGM path costs. More specifically, FIG. 3 relates to restoring data for future processing. As shown in FIG. 3, when reading back from the memory, unavailable SGM path costs after sub-sampling are interpolated before use using bi-linear interpolation. To improve the accuracy of interpolation, local cost guided bi-linear interpolation is also proposed as shown in FIG. 4. FIG. 4 is an embodiment of a local cost guided bi-linear interpolation of unavailable SGM path costs. Local cost (i.e. local block-matching cost) and SGM path cost, which is derived partially from local cost, are closely related. For this reason, SGM path cost interpolation is guided by local cost. Note that other interpolation methods other than bi-linear interpolation, e.g. such as cubic interpolation, may be employed. They can be also modified so that it can be guided by local cost as well.

For higher compression ratio, the disparity-dependent SGM path cost quantization is proposed as shown in FIG. 5. FIG. 5 is an embodiment of a disparity-dependent SGM path cost quantization for each pixel. For guaranteed compression ratio without rate control, fixed quantization is employed. However, quantization step is dependent on disparity range. In the following example, SGM path costs between d_(min) and d_(q1) are not quantized, those between d_(q1) and d_(q2) are quantized by Q step=2, those between d_(q2) and d_(q3) are quantized by Q step=4, and those between d_(q3) and d_(max)−1 are quantized by Q step=8. Note that it is also possible to divide disparity range more finely (e.g. >4 Q steps) and coarsely (e.g. just 1 Q step, i.e. disparity independent). Other quantization steps could be used as well. Quantization can be simply done by right bit shift. For instance, Q=2, 4 and 8 can be done by >>1, >>2 and >>3, respectively. Inverse quantization is done by left bit shift.

FIG. 6 is an embodiment of an overall flow according to this disclosure. As shown in FIG. 6, after processing the pixels in every k-th row, all aggregated SGM path costs are compressed by sub-sampling followed by quantization. Afterwards, it is stored in on-chip memory. Before processing the pixels in every (k+1)-th row, these aggregated SGM path costs are fetched and decompressed by inverse quantization followed by interpolation.

FIG. 7 is an embodiment of a process 700 for SGM path cost compression. The process starts at step 702 and proceeds to step 704. At step 704, the method selects a candidate level of disparity. At step 704, the method 700 determined the sub-sampling ratio. The ratio range is lower for lower disparity and higher for higher disparity. At step 706, the method 700 applies the subsampling based on the disparity and ratio. At step 708, the method 700 determines the quantization step based on disparity level. At step 710, the method 700 performs quantization using the quantization step determined. At step 712, the method 700 stores the intermediate date to memory. At step 714, for future data processing, the intermediate data is restored i.e. using inverse quantization and/or bi-linear interpolation. At step 716, the process ends. The intermediate data may include SGM path costs, and the future processing may include calculating SGM path costs for a subsequent row, column, or diagonal based on the intermediate data. In further examples, the future processing may include summing the SGM path costs to produce aggregate SGM costs, and determining a disparity level (e.g., closeness to lens (further from camera=less disparity)) based on the aggregate SGM costs.

FIG. 8 is a block diagram showing an example automobile system 800 that may implement the SGM cost compression techniques of this disclosure. The automobile 800 includes a processor 802, a memory 804, and cameras 806, 808. Cameras 806, 808 are configured to capture a pair of images (i.e., stereo images). Processor 802 may process the stereo images, and determine a disparity level associated with the images. To determine the disparity level, processor 802 may perform one or more of the semi-global matching (SGM) techniques described in this disclosure. Processor 802 may store compressed, sub-sampled, and/or quantized SGM path costs in memory 804, and retrieved the compressed, sub-sampled, and/or quantized SGM path costs from memory 804 for further processing.

Processor 802 may include one or more microprocessors, one or more microcontrollers, one or more digital signal processors (DSPs), one or more integrated circuits (ICs), or any other processing circuitry. Memory 804 may, in some cases, be an example of a non-transitory computer-readable medium comprising instructions that cause processor 802 to perform one or more of the SGM techniques described in this disclosure.

In some examples, processor 802 may be configured to, for each of a plurality of candidate disparity levels: calculate a plurality of costs (e.g., local pixel costs) for a plurality of pixels in a scan region based on the respective candidate disparity level; calculate a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region based on the costs and the respective candidate disparity level; determine a sub-sampling ratio based on the respective candidate disparity level; sub-sample the SGM path costs based on the determined sub-sampling ratio; and store the sub-sampled SGM path costs for the respective candidate disparity level in a memory.

In further examples, processor 802 may be further configured to, for each of the plurality of paths calculate a plurality of semi-global matching (SGM) path costs for pixels in the respective path of the scan region based on the costs and the respective candidate disparity level; sub-sample the SGM path costs for pixels in the respective path based on the determined sub-sampling ratio; and store the sub-sampled SGM path costs for pixels in the respective path in the memory.

In some examples, a sub-sampling ratio for a set of lower candidate disparity levels is less than a sub-sampling ratio for a set of higher candidate disparity levels. In such examples, a sub-sampling ratio of N is indicative of sub-sampling every Nth SGM path cost in a path. In further examples, for a lowest set of disparity levels, processor 802 may perform no sub-sampling (e.g., sub-sampling at a sub-sampling ratio of 1), and for higher disparity levels, processor may perform sub-sampling (e.g., sub-sampling at a sub-sampling ratio that is greater than 1).

In further examples, processor 802 may retrieve the sub-sampled SGM path costs from the memory; interpolate the retrieved sub-sampled SGM path costs to generate a plurality of reconstructed SGM path costs for the respective candidate disparity level; calculate aggregate SGM costs for the respective candidate disparity level based on the reconstructed SGM path costs; and determine a disparity level for a set of stereo images based on the aggregate SGM cost.

In some examples, processor 802 may, for each of the pixels, sum the SGM path costs that correspond to the respective pixel for each of a plurality of paths. In further examples, processor 802 may interpolate the retrieved sub-sampled SGM path costs using bi-linear interpolation based on a set of weights.

In some examples, processor 802 may interpolate the retrieved sub-sampled SGM path costs based on the costs for the pixels and a set of weights. In such examples, processor 802 may, for each of a plurality of pixels, subtract a cost associated with the respective pixel from a retrieved sub-sampled SGM path cost associated with the respective pixel.

In additional examples, processor 802 may, for each of the candidate disparity levels determine a quantization step size based on the respective candidate disparity level; and quantize the sub-sampled SGM path costs based on the determined quantization step size. The processing circuitry is further configured to store the quantized sub-sampled SGM path costs for the respective candidate disparity level in the memory.

In further examples, processor 802 may for each of the candidate disparity levels retrieve the quantized sub-sampled SGM path costs from the memory; inverse quantize the sub-sampled SGM path costs to generate first reconstructed SGM path costs for the respective candidate disparity level; interpolate the first reconstructed SGM path costs to generate second reconstructed SGM path costs for the respective candidate disparity level; calculate aggregate SGM costs for the respective candidate disparity level based on the second reconstructed SGM path costs; and determine a disparity level for a set of stereo images based on the aggregate SGM cost.

In some examples, processor 802, memory 804, and cameras 806, 808 may form and/or be a part of an Advanced Driver Assistance System (ADAS) of automobile 800.

While the foregoing is directed to embodiments of this disclosure, other and further embodiments may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

What is claimed is:
 1. A method comprising: receiving, by a processor, a stereo image; calculating, by the processor, a plurality of costs for a plurality of pixels in a scan region of the stereo image based on a candidate disparity level; calculating, by the processor, a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region, based on the costs and the candidate disparity level; determining, by the processor, a sub-sampling ratio for SGM path costs to be: a first sub-sampling level in response to determining that the candidate disparity level is in a first range; a second sub-sampling level in response to determining that the candidate disparity level is in a second range; and a third sub-sampling level in response to determining that the candidate disparity level is in a third range; sub-sampling, by the processor, the SGM path costs based on the sub-sampling ratio for SGM path costs, to generate sub-sampled SGM path costs; determining, by the processor, a quantization step size based on the candidate disparity level; quantizing, by the processor, the sub-sampled SGM path costs based on the quantization step size, to generate quantized sub-sampled SGM path costs; and storing, by the processor in memory, the quantized sub-sampled SGM path costs.
 2. The method of claim 1, wherein the path is a first path in a plurality of paths, and wherein the method further comprises, for each of the plurality of paths: calculating SGM path costs for pixels in the respective path of the scan region based on the costs and the candidate disparity level; sub-sampling the SGM path costs for pixels in the respective path based on the sub-sampling ratio; and storing the sub-sampled SGM path costs for pixels in the respective path in the memory.
 3. The method of claim 1, wherein the first sub-sampling level is less than the second sub-sampling level, and wherein disparity levels in the first range are smaller than disparity values in the second range.
 4. The method of claim 1, further comprising: retrieving, by the processor, the sub-sampled SGM path costs from the memory, to generate retrieved sub-sampled SGM path costs; interpolating the retrieved sub-sampled SGM path costs, to generate a reconstructed SGM path costs for the candidate disparity level; calculating aggregate SGM costs for the candidate disparity level based on the reconstructed SGM path costs; and determining a selected disparity level for a set of stereo images based on the aggregate SGM costs.
 5. The method of claim 4, wherein calculating the aggregate SGM costs includes summing the SGM path costs that correspond to a respective pixel for each of a plurality of paths.
 6. The method of claim 4, wherein interpolating the retrieved sub-sampled SGM path costs includes interpolating the retrieved sub-sampled SGM path costs using bi-linear interpolation based on a set of weights.
 7. The method of claim 4, wherein interpolating the retrieved sub-sampled SGM path costs includes interpolating the retrieved sub-sampled SGM path costs based on the costs for the pixels and a set of weights.
 8. The method of claim 7, wherein interpolating the retrieved sub-sampled SGM path costs based on the costs for the pixels includes, for each of the plurality of pixels, subtracting a cost associated with a respective pixel from a retrieved sub-sampled SGM path cost associated with the respective pixel.
 9. The method of claim 1, further comprising: retrieving the quantized sub-sampled SGM path costs from the memory, to generate retrieved quantized sub-sampled SGM path costs; inverse quantizing the retrieved quantized sub-sampled SGM path costs, to generate first reconstructed SGM path costs for the candidate disparity level; interpolating the first reconstructed SGM path costs, to generate second reconstructed SGM path costs for the candidate disparity level; calculating aggregate SGM costs for the candidate disparity level based on the second reconstructed SGM path costs; and determining a selected disparity level for a set of stereo images based on the aggregate SGM costs.
 10. The method of claim 1, wherein determining the quantization step size based on the candidate disparity level comprises: selecting a first quantization level in response to determining that the SGM path costs are in a first SGM path cost range; selecting a second quantization level in response to determining that the SGM path costs are in a second SGM path cost range; selecting a third quantization level in response to determining that the SGM path costs are in a third SGM path cost range; and selecting a fourth quantization level in response to determining that the SGM path costs are in a fourth SGM path cost range.
 11. An apparatus comprising: a processor; and a non-transitory computer readable storage medium comprising instructions that, when executed by the processor, cause the processor to: receive a stereo image; calculate a plurality of costs for a plurality of pixels in a scan region of the stereo image based on a candidate disparity level; calculate a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region based on the costs and the candidate disparity level; determine a sub-sampling ratio for SGM path costs to be: a first sub-sampling level in response to determining that the candidate disparity level is in a first range; a second sub-sampling level in response to determining that the candidate disparity level is in a second range; and a third sub-sampling level in response to determining that the candidate disparity level is in a third range; sub-sample the SGM path costs based on the sub-sampling ratio for SGM path costs, to generate sub-sampled SGM path costs; determine a quantization step size based on the candidate disparity level; quantize the sub-sampled SGM path costs based on the quantization step size, to generate quantized sub-sampled SGM path costs; and store, in memory, the quantized sub-sampled SGM path costs.
 12. The apparatus of claim 11, wherein the path is a first path in a plurality of paths, and wherein the instructions further comprise instructions to: calculate SGM path costs for pixels in the respective path of the scan region based on the costs and the candidate disparity level; sub-sample the SGM path costs for pixels in the respective path based on the sub-sampling ratio; and store the sub-sampled SGM path costs for pixels in the respective path in the memory.
 13. The apparatus of claim 11, wherein the first sub-sampling level is less than the second sub-sampling level, and wherein disparity levels in the first range are smaller than disparity values in the second range.
 14. The apparatus of claim 11, wherein the instructions further comprise instructions to: retrieve the sub-sampled SGM path costs from the memory, to generate retrieved sub-sampled SGM path costs; interpolate the retrieved sub-sampled SGM path costs, to generate a plurality of reconstructed SGM path costs for the candidate disparity level; calculate aggregate SGM costs for the candidate disparity level based on the reconstructed SGM path costs; and determine a selected disparity level for a set of stereo images based on the aggregate SGM costs.
 15. The apparatus of claim 14, wherein the instructions further comprise instructions to sum the SGM path costs that correspond to a respective pixel for each of a plurality of paths.
 16. The apparatus of claim 14, wherein the instructions further comprise instructions to interpolate the retrieved sub-sampled SGM path costs using bi-linear interpolation based on a set of weights.
 17. The apparatus of claim 14, wherein the instructions further comprise instructions to interpolate the retrieved sub-sampled SGM path costs based on the costs for the pixels and a set of weights.
 18. The apparatus of claim 17, wherein the instructions further comprise instructions to, for each of the plurality of pixels, subtract a cost associated with a respective pixel from a retrieved sub-sampled SGM path cost associated with the respective pixel.
 19. The apparatus of claim 11, wherein the instructions further comprise instructions to: retrieve the quantized sub-sampled SGM path costs from the memory, to generate retrieved quantized sub-sampled SGM path costs; inverse quantize the retrieved quantized sub-sampled SGM path costs to generate first reconstructed SGM path costs for the candidate disparity level; interpolate the first reconstructed SGM path costs to generate second reconstructed SGM path costs for the candidate disparity level; calculate aggregate SGM costs for the candidate disparity level based on the second reconstructed SGM path costs; and determine a selected disparity level for a set of stereo images based on the aggregate SGM costs.
 20. The apparatus of claim 11, wherein the apparatus is included in an Advanced Driver Assistance System (ADAS) of an automobile.
 21. A non-transitory computer-readable storage medium comprising instructions that, when executed by a processor, cause the processor to: receive a stereo image; calculate a plurality of costs for a plurality of pixels in a scan region of the stereo image based on a candidate disparity level; calculate a plurality of semi-global matching (SGM) path costs for a subset of the pixels that form a path in the scan region based on the costs and the candidate disparity level; determine a sub-sampling ratio for SGM path costs to be: a first sub-sampling level in response to determining that the candidate disparity level is in a first range; a second sub-sampling level in response to determining that the candidate disparity level is in a second range; and a third sub-sampling level in response to determining that the candidate disparity level is in a third range; sub-sample the SGM path costs based on the sub-sampling ratio for SGM path costs, to generate sub-sampled SGM path costs; determine a quantization step size based on the candidate disparity level; quantize the sub-sampled SGM path costs based on the quantization step size, to generate quantized sub-sampled SGM path costs; and store the sub-sampled SGM path costs.
 22. The method of claim 10, wherein the first quantization level is no quantization, the second quantization level is a quantization step of 2, the third quantization level is a quantization step of 4, and the fourth quantization level is a quantization step of
 8. 